Pneumocystis carinii pneumonia (PCP) is a type of serious lung infection that is common in people with HIV infection when the CD4 counts (a type of infection fighting blood cell destroyed by the HIV virus) are less than 200 cells. PCP can often be prevented by taking one of several preventative medicines. This study wants to see how often PCP occurs in patients who have not had a history of PCP 6 months before entering the study and had a CD4 count of <100 cells/mm3, or did have a history of PCP 6 months before entering the study and a rise in their CD4 cell count to >200 cells/mm3, and have stopped taking medications that help prevent PCP.